Crosslinking, covalent

Recent studies in our laboratories have been concerned with the physical properties of sulfonated ionomers such as sulfonate ethylene/propylene/ethylidene norbornene terpolymers (4, or lightly sulfonated polystyrene (S-PS) (11). These ionomers exhibit pronounced ion pair association (at sulfonate levels > 15 milli-equivalents/100 g polymer) to a degree that they appear crosslinked covalently. These interactions can be dissipated by the addition of a polar additive, thereby showing that such associations are indeed physical and do not arise due to covalent crosslinking. 

Intermolecular crosslink Covalent bonds formed in vivo between a side group of one molecule and a side group of another molecule covalent bonds formed between a side group of one molecule and one end of a bifunctional agent and between a side group of a second molecule and the other end of a bifunctional agent. 

FIGURE 46.11. o-Dc (7)for crosslinked PAN-ES, PAN-CSA (CHCI3), and PAN-CSA (m-cresol) samples (from Ref. [73]). The dashed lines are based upon the quasi-1 D VRH model. Note here crosslinks refers to physical crosslinks (microcrystalline regions) not chemical crosslinks (covalent bonds). 

IPNs A type of polymer blend prepared to modify the properties of NR. They are composed of two or more polymers with at least one being poly-merized/crosslinked in their networks both without and/or with covalent bonds between the chains of the same or different polymer types. They can be categorized into two types non-covalent IPNs and covalent semi-IPNs (Figure 7.1). For the covalent semi-IPNs, the crosslinked covalent bonds occur between the different polymer chains. In addition, non-covalent IPNs can be further categorized into full IPNs and semi-IPNs. Full IPNs are defined as a combination of NR and other polymers in a network in which each is synthesized by polymerization with a crosslinking agent. In semi-IPNs, in contrast, only one type of polymeric component is crosslinked. However, there are no covalent bonds between the chains of the different polymer types, but the chains of the polymer become inserted into the framework of the other polymers. Moreover, pseudo-IPNs are defined as a type of IPN in which one of the... 

In the lightly cross-linked polymers (e.g. the vulcanised rubbers) the main purpose of cross-linking is to prevent the material deforming indefinitely under load. The chains can no longer slide past each other, and flow, in the usual sense of the word, is not possible without rupture of covalent bonds. Between the crosslinks, however, the molecular segments remain flexible. Thus under appropriate conditions of temperature the polymer mass may be rubbery or it may be rigid. It may also be capable of ciystallisation in both the unstressed and the stressed state. 

In these cases, the polymer remains processible in the gelled state, because it is in the form of discrete PSA particles dispersed in the reaction medium. However, once the particles are dried, redispersion may be difficult if strong interactions develop between the particle surfaces. Polymerization of the acrylic PSA directly on the substrate, as in the case of UV polymerization, can also yield a covalently crosslinked polymer that does not require any further coating steps [71]. 

Covalent crosslinking. Acrylic polymers can be covalently crosslinked through direct reaction between functional monomers in the polymer itself or by the addition of a crosslinking reagent, which typically reacts with the functional groups or polymer backbone in the PSA. In general, acrylic polymers are very... 

It may also be possible to crosslink the acrylic PSA with the help of multifunctional acrylates or methacrylates [87], These monomers can simply be copolymerized with the balance of the other monomers to form a covalently crosslinked network in one step. Since the resulting polymer is no longer soluble, this typ)e of crosslinking is typically limited to bulk reactions carried out as an adhesive coating directly on the article or in emulsion polymerizations where the crosslinked particles can be dried to a PSA film. 

TPEs are materials that possess, at normal temperatures, the characteristic resilience and recovery from the extension of crosslinked elastomers and exhibit plastic flow at elevated temperatures. They can be fabricated by the usual techniques such as blow molding, extrusion, injection molding, etc. This effect is associated with certain interchain secondary valence forces of attraction, which have the effect of typical conventional covalent crosslinks, but at elevated temperatures, the secondary... 

The possibility of conformational changes in chains between chemical junctions for weakly crosslinked CP in ionization is confirmed also by the investigation of the kinetic mobility of elements of the reticular structure by polarized luminescence [32, 33]. Polarized luminescence is used for the study of relaxation properties of structural elements with covalently bonded luminescent labels [44,45]. For a microdisperse form of a macroreticular MA-EDMA (2.5 mol% EDMA) copolymer (Fig. 9 a, curves 1 and 2), as compared to linear PM A, the inner structure of chain parts is more stable and the conformational transition is more distinct. A similar kind of dependence is also observed for a weakly crosslinked AA-EDMA (2.5 mol%) copolymer (Fig. 9b, curves 4 and 5). 

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